Creation of a well characterized collection of lethal mutations induced by single P elements insertion was proposed as part of the Drosophila Genome Center Grant on which the current application is based. By defining STS's within sequences adjacent to all those insertions disrupting different vital genes, this collection would serve as a versatile link between the genetic and physical maps of the Drosophila genome. There are many uses for such a resource by the Drosophila research community; furthermore it will directly benefit ongoing efforts at large scale sequencing. The experiments proposed here would increase the number of genes and STS's contained in the collection from approximately 1,100 to at least 1,850. Furthermore, on chromosome 2, where the new lines are concentrated, the representation of vital genes in the collection will increase from 27% to 63% of the estimated total. The fraction of all autosomal lethal complementation groups represented in the collection will increase from 27% to 46%. This improved collection will therefore have even greater utility for the Drosophila research community and will significantly advance the goals of the Drosophila Genome Center. The scale of these whole genome saturation mutagenesis experiments are unprecedented in other multicellular organisms, and are likely to provide unique insights into the organization of eukaryotic genomes.